As semiconductor technology advances and device sizes shrink, the impact of contaminants in the gases used to fabricate these devices becomes increasingly important. Many specialty gases are used in the production of semiconductor devices. For example, a major source of contamination in the chip fabrication process is the presence in process gases of trace amounts e.g., on the order of tens of parts per billion by volume (ppbv) of water. While ultra pure (water and other contaminant free) gases are sometimes available, chemical reactions, phase changes, and other effects often result in the presence of moisture in the gas supply line in a semiconductor fabrication system. These facts make it very attractive to have moisture sensors that are small, efficient, have rapid response times, and are inexpensive enough to be placed in multiple locations in the system. These locations include each line of the gas box and also immediately before the reactor in a semiconductor fabrication system. Currently available sensors do not meet all the aforementioned requirements.
Semiconductor fabs require as many as fifty gases to process a wafer. Table 1 lists some of the gases associated with several different process steps. Contaminants in these gases can diminish yields and degrade chip reliability. One common contaminant is water vapor which may distort manufacturing processes and thereby compromise device performance. Process steps which are vulnerable to moisture contamination include epitaxial growth, sputtering, metal-organic vapor phase epitaxy, thermal etching, gas phase etching of tungsten films, plasma etching of silicon and polysilicon, and chemical vapor deposition of polysilicon, silicon dioxide, silicon nitride, and tungsten. The presence of water can accelerate or retard the chemical reactions which occur during deposition or etching, thereby altering the thickness and/or composition of critical layers. Moisture can create pitting, hazing, and stacking faults; cause resist patterns to fail; induce diodes and junctions to leak; and otherwise degrade the service lifetime of products which pass inspection and reach the marketplace. Purity requirements for process gases will likely become more strict as line widths continue to shrink.
The ambient atmosphere typically contains water vapor concentrations of up to, or even exceeding, approximately 1%. This is at least six orders of magnitude greater than the acceptable limits for process gases. As such, even very small leaks of ambient air into the gas distribution system may introduce significant moisture. Furthermore, even well-dried gas lines may contribute moisture to otherwise high-purity gas. After a gas distribution system is purged with an inert gas, some water molecules can remain in the lines due to strong binding to adsorption sites on metal and oxide surfaces. When polar gases, such as HBr and HCl, enter the lines, they tend to displace water molecules, which enter the gas stream. Additionally, surface oxides within the gas distribution system can interact with corrosive gases to create water molecules by chemical reactions such as the following:Fe2O3+6HCl  2FeCl3+3H2O   (1)Since even dry components within the distribution system can spontaneously create water, it seems clear that the semiconductor industry needs real time, in-line moisture monitors to protect wafer fabrication systems from moisture contamination.
TABLE 1Process Steps and Associated GasesProcess StepsGases (Inert Gases, Hydrides, and Corrosives)OxidationCarriersAr, N2ReactantsCl2, H2, HCl, O2PhotolithographyCarriersAr, N2,EtchingPlasma or ReactiveAr, BCl3, Cl2, CF2, CF4, C2F6,Ion EtchingHe, H2, N2, O2, C2F8, SiF4, SF6CarriersAr, H2, Ne, XeDiffusionCarriersAr, H2, N2, O2Dopant SourcesAsH3, BCl3, B2H6, PH3Chemical VaporOxidationCO2, N2, O2, H2SiCl2, SiH4DepositionDopingAsH3, B2H6, , PH3, SiH4NitrideN2O, SiCl4, NH3III-V LayersAsH3, H2, HCl, H2S, PH3Ion ImplantationAr, AsH3, BCl3, BF3, B11F3, Cl2, He, H2, H2S, N2,PH3, SiH4, P2F6, SiF4AnnealingAr, H2, N2MetallizationArBondingAr, H2, N2Crystal GrowthAr, He, H2EpitaxyCarriersAr, H2, N2Silicon SourcesSiH4, H2SiCl2, HSiCl3, SiCl4DopantsAsH3, B2H6, PH3EtchantHCl
Additionally, moisture will react with certain gases, yielding acids which corrode gas handling equipment. For example, aqueous hydrochloric acid attacks iron and other constituents of stainless steel. As corrosion advances, pipes, valves, mass flow meters, mass flow controllers, and other components can fail, causing equipment downtime. Furthermore, corroded pipes release particles which enter the gas stream. Gas-phase nucleation by particles and flaking of particles from gas lines onto wafers can reduce yield. According to one report, the gas distribution system accounts for 68% of all contamination in CMOS processes. Moisture in arsine and phosphine lines may also contaminate the ultra-high-vacuum chambers used for doping wafers. Water molecules in the chamber can make it impossible to draw a sufficiently high vacuum, forcing engineers to shut down the chamber and subject it to an extended bake.
Manufacturers of LED's and VCSELs generally deposit three to five epitaxial layers by organometallic vapor phase epitaxy, using ultra-high-purity anhydrous ammonia as a process gas. Trace oxygen in the epitaxial layers can limit device performance. The photoluminescence of LED's and VCSELs depend strongly on the moisture content of the ammonia used during production. In order to increase the efficiency, the amount of moisture needs to be accurately monitored during production.
Kermarrec and co-workers at ST Microelectronics and Air Liquide have studied the effect of moisture on SiGe devices. In their words, “A direct correlation between moisture impurity in process gases and atomic oxygen present in epitaxial SiGe layers was demonstrated, both qualitatively and quantitatively. The resulting incorporation of oxygen atoms can induce dislocations into the strained layers, which may degrade device performance and, subsequently, reliability.” O.Kermarrec et.al., Solid Slate Techology, 45(3). Pp.55-60, 2002.
Moisture sensors (monitors) are often referred to as in-line or at-line. Generally, in-line refers to a monitor that is sits in the gas line such that the gas under test passes through the monitor without a need for tapping off the line. The term at-line is generally used for monitors that tap some flow off of the gas line. The flow that is tapped off is generally discarded. Both in-line and at-line monitors for semiconductor industry process gases should advantageously have several characteristics, which are not available with current technologies. The monitor should be sensitive enough to detect moisture in concentrations on the order of 10 ppbv (parts per billion volume) or even lower, be fast enough to react to transient changes in gas flows, compact, and inexpensive enough to be placed at multiple locations in the process train. Since none of the technologies available today can satisfy all of these criteria, it is not surprising that wafer fabs rarely deploy in-line moisture monitors. The present invention satisfies an unmet industry need by providing a system which meets all four requirements (sensitivity, speed, size, and price) and is suitable for both in-line and at-line sensors.
Photoacoustic Spectroscopy (PAS) transforms an optical event into an acoustic event. Gas molecules absorb light at specific, characteristic wavelengths and undergo quantized vibrational or rotational transitions. They gain kinetic energy in the form of heat, and collide with other molecules, creating a pressure wave. Since a pressure wave in a gaseous medium is sound, it can be detected by a microphone. The sensitivity of PAS is determined by the efficiency with which the molecular excitation energy produces a pressure wave and the efficiency with which the pressure wave is converted into an electrical signal.
Alexander Graham Bell discovered the photoacoustic effect in 1881. However, scientific and technological interest in the effect lay dormant for eighty years in the absence of suitable light sources and microphones. In the 1960's, lasers stimulated researchers to explore the photoacoustic effect for spectroscopy. In 1968, Kerr and Atwood detected low concentrations of pollutants in air by using lasers and phase-sensitive, lock-in acoustic detection techniques. (E. L. Kerr, and J. G. Atwood, “The laser illuminated absorptivity spectrophone: a method for measurement of weak absorptivity in gases at laser wavelengths,” Applied Optics, No 7, p. 915-921, 1968. Kreuzer detected methane in nitrogen in 1971, using an intensity-modulated He-Ne laser (L.B. Kreuzer, “Ultralow gas concentration infrared absorption spectroscopy,” J. Applied Physics, Vol. 42, p. 2934-2943, 1971.
In order to further elucidate the photoacoustic effects it is useful to consider the physical steps that result in a photoacoustic signal. The photoacoustic effect in a photoacoustic cell can be divided into four sequential events: 1) absorption of incident optical radiation by a target analyte gas; 2) localized heat release in the sample gas due to transformation of the absorbed light energy into molecular motion; 3) pressure wave generation due to heat induced expansion of the gas; and 4) detection of the pressure wave generated acoustic signal. spectroscopy, and also on optimization of the physical system used to carry out photoacoustic
There is a large body of work on both the theoretical fundamentals of photoacoustic spectroscopy, and also on optimization of the physical system used to carry out photoacoustic spectroscopy. Specifically, it is known that the configuration of the cell in which the gas is contained can influence the detection process. The first cells were simple cylinders with windows at each end which were substantially transparent to the optical excitation beam. An advance in sensitivity was made when it was realized that the optical signal entering the cell could be modulated to induce a pressure wave at an acoustic resonance frequency of the cell, thereby providing a forcing function. This pressure wave can be detected using a microphone attached to the wall of the cell. If the cell dimensions are picked randomly and/or the optical excitation beam has a high overlap with more than one acoustic mode, the result is typically the excitation of higher order modes and/or weak signal strength at the detector.
Today several companies sell PAS systems, which typically consist of a laser light source, an acoustic cell, a sonic transducer to convert a pressure pulse to a voltage pulse, and electronics for digitizing and storing the output signal from the transducer. The more sensitive systems employ large, high-power lasers. Prior art photoacoustic systems for detecting water generally use CO or CO2 lasers. These lasers are expensive, bulky, and require external cooling. Therefore, although existing systems generally meet the requirements of scientific users, they are clearly not suited for use in an industrial in-line or at-line gas sensor configuration.
In 1996, a group at the Hungarian Academy of Sciences reported using a photoacoustic cell placed inside the optical cavity of a diode laser to achieve an order of magnitude gain in detection efficiency compared to extracavity operation. (Z. Bozoki, et.al., Appl Phys., B 63, 399 (1966). The same group later described a PAS system which supplied optical power with a DFB laser (M. Szakáll, Z. Bozôki, A. Mohâcsi, A. Varga, and G. Szabô, “Diode Laser Based Photoacoustic Water Vapor Detection System for Atmospheric Research,” Applied Spectroscopy, Volume 58, Number 7, 792-798, 2004). This system reportedly was able to detect moisture at levels of about 250 ppbv. The system would appear to have come within a factor of about twenty-five of the minimum required sensitivity for semiconductor gases, but did not meet the criteria for industrial use in terms of size or cost. Italian workers (A. Boschetti, et.al., Appl. Phys., B 74, 273-278 (2002), reported use of a pulsed laser for methane and ethylene detection. Additionally, they referred to the earlier Hungarian work and stated “Placing the resonant PA cell inside an external build-up cavity will provide a higher gain while maintaining the possibility of pressure control in the sampling cell.” No further details were given so it is unclear what type of laser or detector configuration they were contemplating.
Clearly, methods and systems for increasing the sensitivity of photoacoustic spectroscopy units, while simultaneously reducing their size and the cost are essential if widespread industrial use is to be realized. Widely deployable systems for industrial use will require small and inexpensive light sources, such as the CW diode lasers heretofore developed for telecommunications use. These lasers are small, relatively inexpensive, and convert electrical energy into optical energy with high efficiency.